Between January 2009 and September 2013, there were nearly two mass shootings per month in the U.S., according to a new report by Mayors Against Illegal Guns. The incidents described in the report include the July 2012 shooting at the Century 16 theater in Aurora, which left twelve dead and seventy wounded.

The report (which is on view below) contains some interesting findings, including that 67 percent of mass shootings happened in private homes, with only 15 percent occurring in public "gun-free" zones.

"Contrary to rhetoric, gun-free zones are not the problem," Mayors Against Illegal Guns's communications director Erika Soto Lamb concludes in a statement.

Other noteworthy statistics from the report, which uses the FBI's definition of a mass shooting as an incident in which at least four people were killed with a gun:

In 57 percent of cases, the shooter killed a spouse or other family member.

In 11 percent of cases, concerns about the shooter's mental health had been brought to the attention of a medical professional, school official or legal authority.

Assault weapons or high-capacity magazines were used in 15 percent of the incidents. But those incidents were among the most deadly, with an average of 14.4 people shot.

The shooter committed suicide in 43 percent of the incidents.

In 14 percent of the shootings, law enforcement or military officers were targeted in the attack or killed or injured while responding to it.

Mayors Against Illegal Guns is among the most vocal national gun-control groups and actively campaigned against the recent recall of Colorado senators John Morse and Angela Giron, prompted by the lawmakers' support of firearms-control bills.

Below, read the group's entire report, which includes descriptions of 93 mass shootings.